1. Field of the Invention
The present invention relates to a nanocrystalline phosphor and a coated nanocrystalline phosphor as well as a method of preparing a coated nanocrystalline phosphor, and more specifically, it relates to a nanocrystalline phosphor improved in luminous intensity and luminous efficiency, a coated nanocrystalline phosphor prepared by coating a nanocrystalline phosphor with modified organic molecules and a method of preparing a coated nanocrystalline phosphor through a simple synthetic procedure with a high synthesis yield.
2. Description of the Background Art
It is known that semiconductor crystallites (hereinafter referred to as “crystallites”) exhibit a quantum size effect when reduced to about the Bohr radius. The quantum size effect is such an effect that electrons confined in a nanometer-sized space remarkably exhibit the property of a wave motion. When a substance is reduced in size, electrons therein cannot freely move and the energy thereof can assume only an unarbitrary specific value in such a state, to result in the quantum size effect. For example, the wavelength of light emitted from crystallites of about the Bohr radius is reduced in proportion to the size (refer to C. B. Murray, D. J. Norris and M. G. Bawendi “Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites”, Journal of the American Chemical Society, 1993, 115, pp. 8706-8715; hereinafter referred to as “Document 1”). However, a phosphor prepared from a group II-VI compound semiconductor described in Document 1 is inferior in reliability and durability, and contains an environmental pollutant such as cadmium or selenium. Therefore, a material substituting for this phosphor has been demanded.
An attempt has been made to synthesize microcrystals of a nitride-based semiconductor as a material substituting for the group II-VI compound semiconductor (refer to Japanese Patent Laying-Open 2000-198978; hereinafter referred to as “Document 2”). According to Document 2, a gallium nitride phosphor is synthesized by preparing gallium oxide from gallium hydroxide and heating this gallium oxide in ammonia. According to Document 2, the heating temperature is about 1000° C., for causing a vapor phase reaction.
Synthesis of GaN crystallites is studied also in a liquid phase reaction (refer to Yi Xie, Yitai Qian, Wenzhong Wang, Shuyuan Zhang and Yuheng Zhang “A Benzene-Thermal Synthetic Route to Nanocrystalline GaN”, SCIENCE, June, 1996, Vol. 272, No. 5270, pp. 1926-1927; hereinafter referred to as “Document 3”). According to Document 3, GaN nanocrystallites are synthesized by reacting gallium trichloride and lithium nitride with each other in a benzene solution.
However, GaN crystallites have an emission wavelength mainly in the near ultraviolet region. Further, the GaN crystallites cannot exhibit a quantum size effect. Therefore, deep studies are conducted in order to implement crystallites of group 13 nitride semiconductor having an emission wavelength in the visible region, and a study for controlling the mixed crystal ratio of crystallites in the group 13 nitride semiconductor is conducted as a part thereof.